This invention relates to nematic liquid crystals in electro-optical components for the nodulation of traversing or reflected light as well as for the representation of numerals, signs and images as well as to a method for producing the same. In view of their electric anisotropy, liquid crystals exhibit a special interaction effect with electric fields and may, therefore, be oriented. Nematic liquid crystals with positive dielectric anisotropy orient themselves parallel to the electric field, whereas those with negative dielectric anisotropy orient themselves vertical to the field. In view of dielectric relaxiation, negative dielectric anisotropy may occur above the relaxation frequency in certain liquid crystals which have positive dielectric anisotropy at low frequencies. In the vicinity of the relaxation frequency (isotropic frequency), dielectric anisotropy equals zero. It has already been proposed to utilize this effect in electro-optical displays by operating the display at two frequencies (DD-WP 107 561).
This type of activation makes it possible to change the on and off switching time solely through a change in the strength of the electric field so that, on normal operation, very slow pulse decays of dielectric information can be considerably shortened. The previously known substances have either an uncomfortably high relaxation frequency (in the MHz range), too high melting temperatures, or yet are chemically and thermally unstable. The previously known 2-subst.-1,4-bis[4-acyloxy]-benzenes (WP 106 933, 108 022, 108 023, 116 732, 139 575, 139 592; also A. C. Griffin et al. Mol. Cryst. Liqu. Cryst. 44, 267 (1978); T. A. Rotinyan et al. Kristallografya 23, 578 (1978)), in which the 2-substituent, however, is rather short, also are ill-suited to the two frequency range in view of their high relaxation frequencies. Therefore, an active search for new substances that do not have these disadvantages, or have them on a very limited scale, continues.
The liquid crystal compounds produced according to the method of this invention are new, no methods for their production being previously known.
The previously known 2-subst.-1,4-bis-[4-acyloxy]-benzenes of the general formula: ##STR2## obtained through the conversion of a corresponding 2-subst. hydroquinone with a corresponding subst. benzoic acid or cyclohexane carboxylic acid or one of its derivatives, have only short-chained substituents in the 2-position on the benzene ring [WP 106 933, 108 022, 108 023, 116 732, 139 575, 139 592; A. C. Griffin, D. L. Wertz, A. C. Griffin Jr.: Mol. Cryst. Liqu. Cryst. 44, 267 (1978); T. A. Rotinyan, Ch. K. Rout, A. P. Kovshik, P. V. Adomenas, Yu. Yu. Dangvila, E. I. Ryumtsev: Kristallogr. 23, 578 (1978)]. However, these substances are ill-suited for two-frequency operations since their relaxation frequencies lie relatively high (in the MHz range).
It is the object of the invention to create new nematic liquid crystals with low relaxation frequencies, low melting temperatures and good chemical and thermal stability which are well-suited for electro-optical devices, preferably for two-frequency operation, as well as methods for producing the same.